1. Field of the Invention
The invention relates to a sintered ceramic for high-stability thermistors based on Ni.sub.x Mn.sub.3-x O.sub.4, where x&gt;0. The invention also relates to a method for the production of such a sintered ceramic.
Ceramic materials of that generic type have also gained importance as thin films in radiation receivers.
British Patent Specification 1 266 789, for instance, discloses technical versions based on semiconducting oxides of the transition elements and combinations thereof, for instance in spinels. Multiphase systems, such as cobalt-manganese-oxide systems, are often used, as modified by further components such as copper oxide, nickel oxide or lithium oxide (see U.S. Pat. No. 3,219,480, for example), without seeking the advantage of forming a uniform phase. A rated resistance R.sub.25 (electrical resistance of the thermistor at T=25.degree. C.), and a B constant that is definitive for the sensitivity of the temperature measurements, according to the equation: EQU R(T)=R.sub.0 exp(B/T)=R.sub.25 exp(1/T-1/298)
is adjusted to variable values, on the basis of such multiphase systems, by suitably carrying out the reaction in the sintering process, so that for a given composition, it is possible to produce a certain assortment of thermistors. That kind of procedure generally exhibits considerable scattering of the data of the individual specimens and in particular from one batch to another, since the electrical parameters that characterize the thermistor assume different values, depending on the microstructure which is attained. In such heterogeneous systems, the equilibrium composition of the phases is generally pressure-dependent, which has negative effects on the stability of the electrical parameters over time.
The publication entitled: Siemens Zeitschrift [Siemens Journal] 47, January 1973, No. 1, pp. 65-67, discloses the making of thermistors based on the Ni.sub.x Mn.sub.3-x O.sub.4 system. In the case of the composition range of 0&lt;x&lt;1.275, a largely uniform phase results, which no longer has the aforementioned disadvantages of pronounced scattering, on the condition that a ceramic microstructure is attained, but that is associated with the use of a special sintering aid. When the sintering process for thermistor production is carried out, heat composition into a heterogeneous mixture must be avoided with those oxide semiconductors that have a uniform phase condition, when the temperature is below 720.degree. C. in the air. The application range is therefore limited to approximately 150.degree. C.
In spinel compounds of the Ni.sub.x Mn.sub.3-x O.sub.4 system, in which in accordance with the general formula: EQU Mg.sub.z NiMn.sub.2-z O.sub.4
a stepwise substitution of manganese with magnesium was performed, it was possible with increasing magnesium content to demonstrate a drop in the temperature at which the oxidative decomposition begins, and to attain complete stability for the composition MgNiMnO.sub.4 (where z=1). That is described in German Published, Non-Prosecuted Application DE 42 13 631 A1, corresponding to U.S. application Ser. No. 08/328,074, filed Oct. 24, 1994.
It is disadvantageous that in the high temperature range which is associated with the deposition of an NiO phase, the temperature of the oxygen splitoff in an O.sub.2 atmosphere is reduced from 975.degree. C. for NiMn.sub.2 O.sub.4 to 680.degree. C. for MgNiMnO.sub.4. In order to attain an adequate sintering densification, the process must therefore be carried out by exceeding the upper limit of stability and must go through a heterogeneous stage. In other words, a homogeneous ceramic is not obtained until a generally time-consuming reoxidation occurs at temperatures of less than 700.degree. C., in which unification of the phases takes place. The disadvantages can be eliminated by incorporating Fe.sup.III into the Ni.sub.x Mn.sub.3-x O.sub.4 system in a EQU Fe.sub.1-y Ni.sub.x Mn.sub.2-x+y O.sub.4 series,
which is performed by generating an equivalent of manganese (III) and at the same time of manganese (IV) that are required for conductivity by incorporating iron at the B sites of the spinel lattice; and for the cubic spinel FeNiMnO.sub.4, with a cation distribution as follows: EQU Fe.sup.III.sub.0.66 Mn.sup.II.sub.0.34 [Ni.sup.II Fe.sup.III.sub.0.34 Mn.sup.III.sub.0.32 Mn.sup.IV.sub.0.34 ]O.sub.4
as derived from Mossbauer measurements, an upper decomposition temperature T.sub.z =1020.degree. C. in air is attained. It has been found that a semiconductor ceramic of that composition is distinguished by a significantly high aging stability. That has been described in European Patent Application 0 609 776, corresponding to U.S. application Ser. No. 08/191,087.